1. Field of the Invention
The present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program, for executing mutual communications among a plurality of wireless stations such as wireless LAN (Local Area Network) communications, and particularly to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program, in which such wireless network is managed by executing asynchronous and direct communications among terminals (random access).
More specifically, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program, in which a wireless network is constructed by decentralized autonomous operation of each communication station without providing a special apparatus serving as a control station, and particularly relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program, in which each communication station executes intermittent receiving/transmitting operation to perform power saving.
2. Description of the Related Art
As a system capable of releasing users from LAN wirings of a wired system, wireless LAN system is attracting most attention. The use of the wireless LAN provides relatively easy movement of communication terminals such as personal computers (PC), because a most part of wiring cables can be omitted in a working area such as an office space. Recently, along with the advance in high-speed and low cost in the wireless LAN, its demand considerably is increasing. In particular, in recent years, the introduction of the personal area network (PAN) was studied for carrying out information communications by constructing a small-area wireless network among a plurality of electronic apparatuses existing around persons. For example, various wireless communication systems and wireless communication apparatuses are standardized using frequency bands, such as 2.4 GHz band or 5 GHz band, requiring no license by the governments.
As general standards regarding the wireless network, the IEEE (The Institute of Electrical and Electronics Engineers) 802.11 standard, (for example, refer to a non-patent document 1) or, HiperLAN/2 standard (for example, refer to Non-Patent Document 2 or 3), the IEEE 302.15.3 standard, the Bluetooth communication, or the like are proposed. In the IEEE 802.11 standard, various wireless communication methods exist such as the IEEE 802.11a standard, the IEEE 802.11b standard, and the like.
Generally, such method is employed in which, in order to construct a local area network with the wireless technology, an apparatus is provided within an area as a control station, which is called “access point” or “coordinator”, to construct a network under the overall control by this control station.
In the wireless network where an access point is arranged, an access control method on the basis of reservation of a transmission band is generally employed in which at first, the transmission band necessary for transmitting information is reserved at the access point and then carries out the usage of a transmission path in order to avoid collisions with an information transmission from other communication apparatuses in the case that the information transmission is carried out from a communication apparatus. In other words, the arrangement of the access point provides synchronous wireless communications where the communication apparatuses within the wireless network obtain synchronization with each other.
However, the asynchronous communication between communication apparatuses at a transmission side and a reception side in the wireless communication system where the access point exists always requires interfacing by the access point for the wireless communications, so that there is a problem that the utilizing efficiency of the transmission line considerably decreases.
On the other hand, as another method for establishing a wireless network “Ad-hoc (Ad-hoc) communication” is proposed in which each of terminals directly, asynchronously, and wirelessly communicates with each other. In particular, in a small-area wireless network constructed with a relatively small number of adjacently located clients, the Ad-hoc communication is recommended in which any terminal can execute direct, and asynchronous wireless communications without any specific access point.
For example, in a wireless LAN system adopting the IEEE 802.11 system, an Ad-hoc mode is prepared in which it operates decentralized-autonomously in the manner of a peer to peer (Peer to Peer) communication without the arrangement of the control station. In this operation mode, when a beacon transmission timing comes, each terminal counts for a random interval. When any beacon from another terminal is not received until the period expires, the terminal transmits a beacon of itself.
In this case, if a communication station always carries out a waiting for reception, its electric power is consumed in vain. In particular, the problem of the power consumption in mobile apparatuses powered by a battery is serious. Thus, a lot of wireless apparatuses employs so-called intermittent transmitting and receiving operations in order to reduce its power consumption.
In a conventional wireless communicating system, generally a method is adopted in which a control station is arranged in a wireless network, and an intermittent receiving operation is defined under the overall control by the control station. More specifically, such a structure in which each communication station requests the control station for the intermittent receiving operation, and then receives a specified timing of the intermittent receiving operation from the control station is employed. When a certain timing comes, the communication station starts the receiving operation to surely receive the signal from the control station.
Alternatively, there is another method of transmitting and receiving operations in which a control station sets a time domain that the control station uses in its own network, and all stations execute the transmitting and receiving operations in synchronism with the time domain.
However, in the conventional intermittent receiving operation as described above, generally, the communication station as the control station manages the intermittent transmitting and receiving operations of other communication apparatuses, so that there is a problem that the intermittent transmitting and receiving operations cannot be set in the decentralized-autonomous type of communication systems where no control station exists.
In particular, in the case that a lot of frequencies for the intermittent transmitting and receiving operations are prepared, and the operations are set every time of the transmission and reception in accordance with a frequency of the data transmission and reception, the control station should hold an operation condition of the respective communication station, so that a load regarding the network management on the control station becomes excessively large.
Further, in the case of changing the frequency of the intermittent transmitting and receiving operations, the frequency has to be changed with synchronization in the same system, otherwise, the wireless communication apparatuses included in the system cannot operate.
On the other hand, in the decentralized-autonomous type of wireless communication system in which no control station is arranged, it is difficult to set the intermittent transmitting and receiving operations, so that the operations are required in a condition always allowing transmitting and receiving operations irrespective of the communication frequency. This results useless power consumption.
Non-Patent Document 1
International Standard ISO/IEC8802-11: 1999(E) ANSI/IEEE Std. 802.11, 1999 Edition, Part11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.
Non-Patent Document 2
ETSI Standard ETSI TS 101 761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions.
Non-Patent Document 3
ETSI TS 101 761-2 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part2: Radio Link Control (RLC) sublayer.